function getOne(obj) {
    var str = ''
    for (var key in obj) {
        str += `${key}=${obj[key]}&`
    }
    return str.slice(0, str.length - 1)
}

function ajax(method, url, params) {

    return new Promise((resolve, reject) => {



        //1.创建一个xhr对象
        var xhr = new XMLHttpRequest()

        //2.配置请求对象
        xhr.open(method, method == 'get' ? url + "?" + getOne(params) : url)

        //3.设置回调函数
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                resolve(JSON.parse(xhr.responseText))
            } 
        }
        //4.发送请求
        xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded')
        xhr.send(method == 'get' ? null : getOne(params))
    })
}
